System and Method for Measuring Body Joint Range of Motion

ABSTRACT

A system and method are provided for measuring body joint range of motion using inertial measurement unit (IMU) sensors capable of measuring their orientations relative to Earth. The method mounts a primary IMU sensor on a proximal body segment, and a secondary IMU sensor on a distal body segment connected to the proximal body segment via a joint. The sensor orientation relationship is measured between the primary IMU sensor and the secondary IMU sensor. In one aspect, the sensor orientation relationship is expressed as a quaternion. The sensor orientation relationship is converted into at least one physiologically modeled constituent joint rotation relationship between the proximal body segment and the distal body segment. More explicitly, the sensor orientation relationship is converted to a constituent axial axis, composite radial, or primary and secondary radial rotations of the distal body segment measured with respect to the proximal body segment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to human body joint motion evaluationand, more particularly, to a system and method for converting inertialmeasurement unit (IMU) sensor measurements into physiologically definedjoint rotation measurements in a standard format more easily understoodby health care professionals.

2. Description of the Related Art

Measuring a body joint's range of motion (RoM) is critical forapplications such as joint surgery recovery and sports techniquecoaching. It is also important for orthopedic patients to begin flexingthe repaired joint soon after surgery is completed. Often the patientdoes not understand or correctly comply with the doctor's recoveryinstructions, and upon returning to their home, the patient is notsufficiently monitored by medical staff to determine if recovery isproceeding well. This lack of monitoring may result in poor RoM and theneed for additional surgeries. It would also be advantageous to monitorrange of motion dynamically, during normal daily activities. Manyexisting systems only measure statically, when the user is still andfocused specifically on the measurement process.

Currently existing systems do not use physiologically modeled jointrotation decomposition to determine constituent joint rotations. Theyalso are only capable of displaying a single planar rotation, and cannotcalculate or display each of the constituent rotations that makeup thecomposite rotation. They require trained professionals to perform themeasurement. They do not automatically adapt their analysis to thespecific joints that are being measured. They do not show trending overtime during normal daily usage. They are not able to send trendinginformation to remote viewers. Many existing systems only measurestatically, when the user is still and focused specifically on themeasurement process.

It would also be advantageous to monitor range of motion dynamically,during normal daily activities, and present the measurements in anintuitive, easy to understand format.

SUMMARY OF THE INVENTION

The system described herein provides a method for measuring the range ofmotion of body joints when the body segments connected by the joint havean inertial measurement unit (IMU) attached to them reporting theorientation of the IMU relative to a reference object which emitsgravitational and magnetic fields, such as Earth. This system providesan easy to use method to dynamically measure a user's range of motionand report the results back to medical staff or other interested partiesfor monitoring. The user interface adapts to the specific joint beingmeasured. Results are based upon physiologically modeled quaternionjoint rotation decomposition for superior stability and medicallyphysiological relevancy, particularly when compared to Euler angles, yetthe presentation is still intuitive and easily understood by bothmedical personnel and patients.

The system receives body segment spatial orientation vectors real-timefrom a pair of sensors attached to adjoining body segments, calculatesthe rotation of the intersecting joint, separates the joint rotationinto constituent rotations, and displays the results both numericallyand graphically. The user interface enables users to review the statusof the sensors, assign sensors to the desired body segments, and selectthe joint whose range of motion will be monitored. From the jointselection, the system automatically determines the associated distal andproximal body segments and adjusts the display contents to the relevantmetrics for the selected joint.

The display metrics may include a numerical readout, a dial readout, anda long term trending readout of each rotation of relevance for thetarget joint. The readouts include user definable, temporally adjusting,limits and targets for joint range of motion to clearly indicate to theuser if they are approaching their joint rotation targets or exceedingtheir limits. The system can run on a personal computer (PC),smartphone, or smart tablet. Data can be transferred to the worldwideweb for viewing by remote parties.

Accordingly, a method is provided for measuring body joint range ofmotion using IMU sensors capable of measuring their orientationsrelative to Earth. The method mounts a primary IMU sensor on a proximalbody segment, and a secondary IMU sensor on a distal body segmentconnected to the proximal body segment via a joint. The sensororientation relationship is measured between the primary IMU sensor andthe secondary IMU sensor. In one aspect, the sensor orientationrelationship is expressed as a quaternion. The sensor orientationrelationship is converted into at least one physiologically modeledconstituent joint rotation relationship between the proximal bodysegment and the distal body segment. A constituent joint rotationrelationship is defined in no more than two dimensions ofthree-dimensional space. More explicitly, converting the sensororientation relationship into the physiologically modeled constituentjoint rotation relationship includes defining a constituent axial axisrotation of the distal body segment measured with respect to theproximal body segment. An axial axis intersects the joint connecting thedistal body segment to the proximal body segment and is parallel to amajor axis of a smallest rectangular cuboid enclosing a body segment. Amajor axis is defined as a line passing through the center of twoopposing sides of the cuboid that are farthest apart from each other.

In one aspect, the sensor orientation relationship conversion defines aconstituent radial axis composite rotation of the distal body segmentmeasured with respect to the proximal body segment. The radial axisintersects the joint connecting the distal body segment to the proximalbody segment, is orthogonal to an axial axis of the distal body segment,and is orthogonal to an axial axis of the proximal body segment. Inanother aspect, the radial axis composite rotation defines a constituentprimary radial rotation and a constituent secondary radial rotation. Theprimary radial rotation represents a first angle measured with respectto a first axis aligned with the radial axis of maximum possiblephysiologically possible radial rotation. The secondary rotationrepresents a second angle measured with respect to a second axisorthogonal to the first axis and the axial axis.

The method may display the physiologically modeled constituent jointrotation relationship between the proximal body segment and the distalbody segment, show the constituent joint rotation relationship as anumerical value representing an extent of rotation, or map at least oneconstituent rotation relationship to a physiologically defined jointrotation.

Additional details of the above described method, a system for measuringbody joint range of motion using IMUs, and an associated graphic userinterface (GUI) are presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of system for measuring body jointrange of motion using inertial measurement unit (IMU) sensors capable ofmeasuring their orientation relative to Earth.

FIGS. 2A through 2E are figures defining joint composite and constituentrotation relationships.

FIG. 3 depicts meters or dials measuring physiologically modeledconstituent joint rotation relationships as a numerical valuerepresenting an extent of rotation.

FIG. 4 depicts exemplary reference sensor orientations.

FIG. 5 is a diagram depicting a system of Euler angles (prior art).

FIG. 6 is a chart listing the physiological rotations represented by theaxial, primary radial, and secondary radial rotations.

FIG. 7 is a medial diagram of joint movement.

FIGS. 8A and 8B depict an exemplary user interface.

FIG. 9 is a flowchart illustrating a method for measuring body jointrange of motion using IMU sensors capable of measuring theirorientations relative to Earth.

DETAILED DESCRIPTION

The systems and methods described herein use the notation, concepts, andtechniques described in the following applications, which areincorporated herein by reference:

Ser. No. 14/873,946, entitled, SYSTEM AND METHOD FOR DETERMINING THEORIENTATION OF AN INERTIAL MEASUREMENT UNIT (IMU), filed Oct. 2, 2015;

Ser. No. 15/091,869, entitled, SYSTEM AND METHOD FOR DETERMININGORIENTATION OF BODY SEGMENTS USING INERTIAL MEASUREMENT UNITS, filedApr. 6, 2016;

Ser. No. 14/742,852, SENSOR CALIBRATION METHOD AND SYSTEM, filed Jun.18, 2015; and,

Ser. No. 14/707,194, METHOD AND SYSTEM FOR WIRELESS TRANSMISSION OFQUATERNIONS, filed May 8, 2015.

FIG. 1 is a schematic block diagram of system for measuring body jointrange of motion using inertial measurement unit (IMU) sensors capable ofmeasuring their orientation relative to Earth. The system 100 comprisesa primary IMU sensor 102, having a wireless interface 104, mounting on aproximal body segment 106. A secondary IMU sensor 108 has a wirelessinterface 110, and is mounted on a distal body segment 112 connected tothe proximal body segment 106 via a joint 114. As shown in this figure,the upper arm is the proximal body segment 106 and the forearm is thedistal body segment 112.

A joint rotation measurement device 116 comprises a processor 118, anon-transitory memory 120, a communication module 122 having a wirelessinterface 124 to communicate with the primary IMU sensor 102 andsecondary IMU sensor, and an internal interface, which may be adata/address bus 126 if all the elements are co-located. A conversionapplication 128 is connected to the internal interface 126, and isenabled as a sequence of processor executable instructions, and storedin the memory 120. The conversion application 128 measures the sensororientation relationship between the primary IMU sensor 102 and thesecondary IMU sensor 108, and converts the sensor orientationrelationship to supply, via input/output (IO) 130, physiologicallymodeled constituent joint rotation relationship data describing therelationship between the proximal body segment 106 and the distal bodysegment 112. Typically, the conversion application 128 measures thesensor orientation relationship by expressing the sensor orientationrelationship as a quaternion. A constituent joint rotation relationshipis defined in no more than two dimensions of three-dimensional space,and is explained in greater detail below.

As shown, the IMU outputs are enabled as wireless devices, however, insome circumstances the output may be a hardwired or be an opticalinterface. The figure also implies that the processor 118, memory 120,and conversion application 128 reside in an external device—the jointrotation measurement device 116. For example, the joint rotationmeasurement device 116 may be a smartphone, personal computer, orstand-alone device. However, in some aspects, some or all of theelements of the processor 118, memory 120, and conversion application128 reside in an IMU or in a network-connected server or database.

FIGS. 2A through 2E are figures defining joint composite and constituentrotation relationships. In FIG. 2A, the relationship between theproximal body segment and distal body segment 112 can be understood asquaternion. A quaternion is a 4-element vector (r, x, y, z) based onrotation angle θ and unit length rotation axis {right arrow over (u)}. Aquaternion that rotates an object from spatial coordinate system A tospatial coordinate system B is defined as the normalized vector _(A)^(B)Q.

 _(A)^(B)Q = (r, x, y, z) = r + îx + ĵy + k̂zî² = ĵ² = k̂² = îĵk̂ = −1${r = {\cos ( \frac{\theta}{2} )}};{x = {u_{x}{\sin ( \frac{\theta}{2} )}}};{y = {u_{y}{\sin ( \frac{\theta}{2} )}}};{z = {u_{z}{\sin ( \frac{\theta}{2} )}}}$r² + x² + y² + z² = 1.

In this example,

J=joint Composite Rotation

û=(0.67, 0.48, 0.56)

θ=57.8°.

While the joint composite rotation is accurate, its format isnon-conventional in the medical field, and is better understood whenbroken down into sub-components or constituent rotation relationships.

As shown in FIG. 2B, in one aspect the conversion application suppliesdata defining a constituent axial axis rotation of the distal bodysegment 112 measured with respect to the proximal body segment 106. Anaxial axis intersects the joint connecting the distal body segment tothe proximal body segment and is parallel to a major axis of a smallestrectangular cuboid enclosing a body segment. The major axis is a linepassing through the center of two opposing sides of the cuboid that arefarthest apart from each other. In this case, the axial axis and majoraxes of both body segments are aligned together, so the major axis isnot labeled. The methods disclosed herein assign the axial axis to theY-axis.

In this example:

J_(y)=Constituent Axial Rotation

û=(0, 1, 0)

θ=30°.

In FIG. 2C the conversion application supplies data defining aconstituent radial axis composite rotation of the distal body segmentmeasured with respect to the proximal body segment. The radial axisintersects the joint 114 connecting the distal body segment 112 to theproximal body segment 106, is orthogonal to an axial axis of theproximal body segment, and is orthogonal to an axial axis of the distalbody segment.

In this example:

J_(zx)=Constituent Radial Composite Rotation

û=(0.91, 0, 0.42)

θ=50°.

As an alternative to the constituent radial composite rotation, theconversion application may supply data defining a constituent primaryradial rotation (FIG. 2D) and a constituent secondary radial rotation(FIG. 2E). The primary radial rotation represents a first angle measuredwith respect to a first axis aligned with the radial axis of maximumpossible physiologically possible radial rotation. The methods disclosedherein assign this first axis to the X-axis.

The fact that in this example the joint also happens to be currentlybent the most in the axis of maximum possible radial rotation is just bychance. For example, imagine the neck “joint”. The head can move themost in the forward direction, compared to tipping to the side. So, theprimary radial axis of neck joint goes from one shoulder to the other.The secondary goes from the back of the neck to the front. Even if aperson only tips their head to the side, and not forward or back, theprimary axis still goes from one shoulder to the other, independent ofhow they currently have their head tipped.

In this example:

J_(x)=Constituent Radial Primary Rotation

û=(1, 0, 0)

θ=45°.

The secondary rotation represents a second angle measured with respectto a second axis orthogonal to the first axis and the axial axis. Themethods disclosed herein assign this second axis to the Z-axis. In thisexample:

J_(z)=Contituent Radial Secondary Rotation

û=(0, 0, 1)

θ=20°.

Returning to FIG. 1, a user interface (UI) 132, such as a display, istypically connected to the conversion application 128 on line 134, viaIO 130, to display information representing the physiologically modeledconstituent joint rotation relationship between the proximal bodysegment and the distal body segment. For example, the physiologicallymodeled constituent joint rotation relationships may be depicted withthe three-dimensional cylindrical objects shown in FIGS. 2B-2E, or inthe shape of a realistic human looking avatar. The UI 132 may alsoincorporate a touchscreen with selectable menus, keyboard, mouse,joystick, cursor control device, trackball, stylus, cursor directionkeys, or accept voice commands, that permit a user to enter selectionsand commands.

The IO port 130 may incorporate a modem, an Ethernet card, or any otherappropriate data communications device such as USB. The physicalcommunication links may be optical, wired, or wireless. The jointrotation measurement device 116 may be considered a type of specialpurpose computing system, and as such, can be programmed, configured,and/or otherwise designed to comply with one or more networkingprotocols.

The joint rotation measurement device 116 may provide a directconnection to a remote server via a direct link to a network, such asthe Internet. Connection may be provided through, for example, a localarea network (such as an Ethernet network), a personal area network, awide area network, a private network (e.g., a virtual private network),a telephone or cable network, a cellular telephone connection, asatellite data connection, or any other suitable connection.

FIG. 3 depicts meters or dials measuring physiologically modeledconstituent joint rotation relationships as a numerical valuerepresenting an extent of rotation. These meter or dial readouts may bepresented by the UI of FIG. 1 as an alternative, or in addition to thedepiction of actual body segments. Using neck rotations as an example,then the proximal body segment is the upper chest, and the distal bodysegment is the head. In this case, using medical terminology, meter Adescribes the neck's flexion and extension, meter B describes the neck'slateral bending, meter C describes the neck's rotation, and meter Ddescribes the neck's combined flexion, extension, and lateral bending.

FIG. 4 depicts exemplary reference sensor orientations. An explanationof determining body segment orientations from IMUs attached to thosebody segments can be found in parent application Ser. No. 15/091,869.Since the relationship between adjoining body segments is expressed asconstituent rotation relationships, it becomes relatively easy to selectorientations and viewing angle, which can be graphical represented,based on the selection of the distal body segment. This data permits thedisplay of joint angular rotations in short and long term trendingrepresentations, or the temporary setting of joint rotation targets andlimits. The system permits the viewing of the connection and calibrationstatus of sensors, assigns sensors to a particular body segment, andinitiates the calibration of sensors. Sensors can be identified byrepresenting sensor orientations with corresponding avatar body segmentorientations. The system also permits sending a message to a sensorinstructing it to blink its status LED to aid in identifying whichsensor is associated with which body segment. The conversion applicationmay erase current history data from the display, initiate sensoralignment, and then begin displaying newly received joint rotationsagain.

A quaternion representation of the joint rotation is derived fromadjoining distal and proximal body segment orientations using thefollowing equations.

_(p) ^(d) J=B′ _(p) B _(d)=joint rotation between distal and proximalsegments

-   -   B_(d)=distal segment orientation    -   B_(p)=proximal segment orientation

To compensate for joint rotation calculation offsets that are caused byadjoining body segments having dissimilar reference sensor orientations(such as the elbow joint), the reference sensor offsets are removed fromthe body segment orientations before the joint rotation calculation byrotating them by the inverse of their reference sensor poseorientations.

J _(pc)=(_(E) ^(P) B′ _(p) B _(p))′(_(E) ^(P) B′ _(d) B _(d))=B′ _(pE)^(P) B _(pE) ^(P) B′ _(d) B _(d)

-   -   J_(pc)=pose compenated joint rotation    -   _(E) ^(P)B_(p)=proximal body segment pose orientation    -   _(E) ^(P)B_(d)=distal body segment pose orientation

The pose compensated joint rotation is then positive definite adjustedabout its r dimension to obtain the composite joint rotation of thedistal body segment relative to the proximal body segment.

J=PosDef(r,J _(pc))

In general, the joint composite rotation J does not have anyphysiologically meaningful interpretation. The axis of rotation does notalign with any body segment axis, and the angle of rotation is acombination of multiple constituent rotations. However, using thetechnique described in Ser. No. 15/091,869, this rotation is decomposedinto its axial and radial constituent rotations, which do havephysiologically meaningful interpretations.

J→(J _(zx) ,J _(y))=(r _(zx) ,x _(zx),0,z _(zx)),(r _(y),0,y _(y),0))

-   -   J_(y)=axial rotation    -   J_(zx)=radial rotation

The radial rotation may contain an offset due to the dissimilar Y-axisrotations in the poses of the distal and proximal body segments. Thatrotation is removed by rotating the radial rotation by the pose axialoffsets to obtain the final radial rotation J_(zx,f).

J _(zx,f)=(_(E) ^(P) B′ _(pE) ^(P) B _(d))_(y) J _(zx)(_(E) ^(P) B′_(pE) ^(P) B _(d))_(y)′=(r _(zx,f) ,x _(zx,f),0,z _(zx,f))

From these constituent rotations, the angular values of the joint'saxial, composite radial, primary radial, and secondary radial rotationsare calculated as follows.

$\theta_{a} = {{\frac{360}{\pi}{\arcsin ( y_{y} )}} = {{axial}\mspace{14mu} {rotation}\mspace{14mu} {angle}}}$$\theta_{rp} = {{\frac{360}{\pi}{\arcsin ( x_{{zx},f} )}} = {{radial}\mspace{14mu} {primary}\mspace{14mu} {rotation}\mspace{14mu} {angle}}}$$\theta_{rs} = {{\frac{360}{\pi}{\arcsin ( z_{{zx},f} )}} = {{radial}\mspace{14mu} {secondary}\mspace{14mu} {rotation}\mspace{14mu} {angle}}}$$\theta_{rc} = {{\frac{360}{\pi}{\arccos ( r_{{zx},f} )}} = {{radial}\mspace{14mu} {composite}\mspace{14mu} {rotation}\mspace{14mu} {angle}}}$

FIGS. 2B-2E depict an example joint composite rotation and theconstituent rotations derived from it. First, J is decomposed into theconstituent rotations J_(y) and J_(zx). J_(y) is the distal bodysegment's axial rotation relative to the proximal segment. Next, J_(zx)is mapped into the constituent rotations J_(x) and J_(z) for easiervisualization.

$J_{x} = ( {{\cos ( \frac{\theta_{rp}}{2} )},{\sin ( \frac{\theta_{rp}}{2} )},0,0} )$$J_{z} = ( {{\cos ( \frac{\theta_{rs}}{2} )},0,0,{\sin ( \frac{\theta_{rs}}{2} )}} )$

Note that in general:

J _(zx) ≠J _(z) J _(x)

J _(zx) ≠J _(x) J _(z)

J _(z) J _(x) ≠J _(x) J _(z)

The joint metrics can be presented to a sensor wearer, medicalproviders, social network, athletic coaches, etc.

FIG. 5 is a diagram depicting a system of Euler angles (prior art). Someconventional approaches convert quaternions to Euler angles to determinean angular representation of a rotation.

J→(φ,θ,ψ)

However, this methodology is unstable and subject to gimbal lock. Also,it does not apply a physiological model to the angular derivation, sothe results can easily deviate from physiological reality.

Stable and physiologically meaningful joint rotation angles are producedby using the technique disclosed in Ser. No. 15/091,869 to decompose thejoint rotation into separate axial and composite radial rotations. Also,the radial rotation can be further decomposed into primary and secondaryradial rotations. The orientation of the primary radial rotation of ajoint is orientation of the joint's greatest physiologically possiblerange of motion. For example, the knee, elbow, or neck joint's flexionand extension motions. The secondary radial rotation of a joint isperpendicular to the primary radial rotation and to the axial rotation.For example, the knee and elbow have none, and the neck has lateralbending. An example of composite radial rotation is the necksimultaneously bending both forward and to the side.

In contrast, Euler angle representations do not represent physiologicalrotations well. The Euler angle θ represents the composite radialrotation. The Euler angles φ and ψ do not represent any physiologicalrotation, as they both contribute to the axial rotation, but in general,neither one by itself is the axial rotation, and they degenerate duringgimbal lock to produce highly erroneous axial rotation results.

FIG. 6 is a chart listing the physiological rotations represented by theaxial, primary radial, and secondary radial rotations. The calculatedconstituent angles found using the system disclosed herein are thendirectly mapped to physiologically meaningful joint rotations based uponthe joint being measured. The radial composite rotation is the totalradial rotation, independent of the axes of the primary and secondaryradial rotations.

FIG. 7 is a medial diagram of exemplary joint movements. Neck and backmovements are shown. However, it should be understood that equivalentdiagrams and nomenclature exists for all the joints listed in FIG. 6.Table 1 lists the medical definitions of joint rotations.

The coordinate system of Ser. No. 15/091,869, also useful in the systemdescribed herein, has each body segment's axial axis aligned with thesegment's Y-axis and pointing downward for all segments except the feet,for which the Y-axis points forward. The primary radial axis is alignedwith the body segment's X-axis and points to the figure's right forsouth and top oriented sensors, back for west oriented sensors, andfront for east oriented sensors.

Medical rotations are only expressed in positive angular values, even ifthe joint can rotate in both directions from the 0 angle, such as thehead's ability to tip both forward and backward. Instead of usingpositive and negative signs to distinguish between the two directions,medical convention is to use different angle names, such as flexionversus extension or adduction versus abduction. The system describedherein maps signed angular values to the medical terminology as shown inTable 1.

TABLE 1 Motion Sign Definition Flexion + Bending of a joint so as tobring together the parts it connects Extension − Straightening of ajoint Adduction + Movement that is towards the mid line of the bodyAbduction − Movement that is away from the mid line of the body RotationRight + Movement of a bone around a longitudinal axis Rotation Left −Movement of a bone around a longitudinal axis Medial Rotation + Movementin which the anterior aspect of a limb turns towards the mid lineLateral Rotation − Movement in which the anterior aspect of a limb turnsaway from the mid line Pronation + Forearm rotated so that the palm ofthe hand faces backwards Supination − Forearm rotated so that the palmof the hand faces forwards Inversion + Turning inwards of the plantaraspect of the foot Eversion − Turning outwards of the plantar aspect ofthe foot Dorsiflexion + Movement of the ankle which pulls the toes andfoot up towards the body Plantarflexion − Pointing of the toes and footaway from the body Radial Deviation + Wrist movement that is towards themid line of the body Ulnar Deviation − Wrist movement that is away fromthe mid line of the body

The clockwise versus counter-clockwise rotational orientation formedical rotations generally flips from one side of the body to theother. For example, the right leg abducts outward in a clockwiseorientation relative to a rotational axis pointing forward from thebody, but the left leg abducts outward in a counter-clockwiseorientation relative to that same axis. To account for this, thecalculated axial, primary radial, and secondary radial rotation anglesare multiplied by sign values based upon the joint being measured. FIG.6 lists the sign values.

The user interface allows the user to select the joint being measured.Numerous selection methods are possible. Two examples are drop downmenus and radio buttons. Selection of the joint automatically selectsthe sensors of the joint's distal and proximal body segments. FIG. 6lists the distal and proximal body segments for each joint.

The user interface may include a representative image of the proximaland distal body segments. The proximal segment is drawn without moving.Its orientation is user selectable. The options are vertical,horizontal, or nearest axis. Nearest axis mode automatically sets theorientation to either vertical or horizontal, whichever is closest tothe body segment's actual orientation. The view can be selectively setto front, back, left, right, top, or bottom. Selective setting can beaccomplished by drop down menus, radio buttons, or other means. Thedistal segment is drawn as it moves relative to the proximal segment.The segments are drawn as multi-sided polygonal cylinders, with eachside having a unique color to aid in viewing joint axial rotations. Thedistal segment can be drawn using either the pose uncompensated (_(p)^(d)J) or pose compensated (J_(pc)) joint rotations.

FIGS. 8A and 8B depict an exemplary user interface. The calculatedrotational angles may be displayed on the user interface. There arenumerous methods possible for displaying the calculated data. Eachmethod is designed for easiest consumption of the data by both medicaland non-medical personnel.

First, a numeric value is displayed for each of the four constituentangles. This provides the user with the greatest possible numeric dataresolution. That value is derived from the most recent sensor readings,and can be the last calculated rotation or a filtered value of the mostrecent rotations, such as short term average.

Second, a dial is displayed for each of the four constituent angles.They needle graphically shows the same value as the numeric valuedescribed earlier. Each dial provides numerically labeled graticulemarkings for easy visual reading, and also medical rotational labelssuch as “Extension” and “Flexion” to clearly distinguish the context ofthe dial. The dial label texts automatically change based upon whichjoint is being measured, using the rotational angle names described inthe explanation of FIGS. 2B-2E.

Each dial shows a first indication area. This area indicates the rangeof angles measured over an immediately previous short time period (forexample 5 seconds) and spans between the minimum and maximum anglesmeasured by the dial in that time. Then the user can quickly see thejoint's most recent range of motion measured by each dial.

Each dial also shows a second indication area. This area indicates thetarget range of motion that the user should try to achieve. After jointsurgery, a patient is often requested to slowly build up their range ofmotion. This indicator helps the user quickly see how well they'remeeting their target RoM. The minimum and maximum angles that the targetarea spans can be set using two different methods. First, it can beautomatically updated by a database of targets stored as a function oftime. Using this method, the user always knows their current RoMtargets. The database values can be set by a medical professional,athletic trainer, or other expert, and tailored to the specific user'starget RoM exercise regimen. Secondly, the targets can be manually setusing the slider bars located underneath each dial. This allows an easyoverride of the time based values. A user available setting, such as aradio button, can be used to switch between the automated and manualmodes.

In addition to a target RoM being displayed on the dial, a limit RoMarea can also be set and displayed in a similar manner. The limit RoMdisplays the absolute minimum and maximum angles that the user shouldnot exceed. This would be used, for example, to help keep the user fromover extending joints too quickly after joint surgery.

The user interface can have a “Freeze” button which stops the numericvalue and dials from updating, so that a user can view a recentmovement's RoM for an extended period of time while moving to adifferent bodily position. The Freeze button toggles between on and off.

While the dials are used for short term RoM status, the user interfacemay also contain graphs which display longer term RoM status, to helpusers track and monitor longer term trends. A separate graph is used foreach of the constituent rotations. The vertical axis is the RoMdisplayed in degrees, and the horizontal axis is the time when themeasurement occurred, displayed in minutes, hours, days, or other timeunits. The graphs' time bases are all aligned with each other and havemutually tracking selectable ranges.

Similar to the dials, the graphs display the user's range of motion, theRoM targets, and the RoM limits. The target and limit values are set bythe database. The time base is sub-divided into smaller elements whosetime width is dependent upon the range of the displayed overall timebase. For example, a 14 day time base may have graticules equal to oneday, with each day further divided into elements 15 minutes wide.

Each element shows the RoM for its associated period of time, indicatedby the timeline displayed under the graphs. The minimum and maximumvalues for the user's RoM can be calculated using a number of methods.For example, they can be the minimum and maximum values that occurwithin the elements beginning and ending time. In another example theycan be the separately low-passed filtered values of the minimum andmaximum values of past times, using any desired filter configuration.

The user interface can support transport controls, allowing the user tonavigate to any time location in the data and then select the desiredgraph time base for data viewing. This is particularly useful when datais read from a storage media as opposed to real-time. An example oftransport control is sliding a cursor along the graph time base and thenpanning, zooming in, or zooming out the display, keeping it centered onthe cursor. All data display formats, numeric, dials, and graphs updateas the cursor is moved.

The sensors on either side of the joint being measured need to havetheir values processed by an alignment algorithm for the most accuratejoint rotation calculation. Ser. No. 15/091,869 describes numerousalignment techniques. The user interface allows the user to select theiralignment method of choice. An automatic alignment method selection isalso available. It automatically selects one of the alignment methodsdescribed in Ser. No. 15/091,869 based upon the joint being measured,and displays that selection to the user. The constituent joint rotationrelationship displayed is automatically selected based upon the jointbeing measured. The two methods that may be used are “Arbitrary PoseMethod 3: Arbitrary pose, musculoskeletal model” (AP) and “Known PoseMethod 3: Predetermined pose, unknown direction, predetermined move”(PDM). AP is the easiest method for users and is selected for jointswhere either the distal body segment is highly deterministic (such ashand or foot), or where the distal body segment does not have asecondary radially rotation (such as forearm or shank).

The user interface provides methods for assigning a sensor to aparticular body segment. As described in Ser. No. 14/707,194, eachsensor is assigned a unique ID (0 to 14). Each ID represents a uniquebody segment, as shown in the table of FIG. 4. Assigning a sensor to abody segment therefore reduces to updating a sensor's ID number. Theuser interface has a button to unassign a sensor. This button causes amessage to be sent to a sensor with an existing ID, instructing thatsensor to change its ID to the value 15, which is the value of anunassigned sensor. The user interface also has a button to assign asensor. This button causes a message to be sent to a sensor thatcurrently has and ID of 15, instructing that sensor to change its ID tothe desired body segment's value, thus assigning that sensor to the newbody segment.

The user interface may display the connection and calibration status foreach sensor. The connection status displays whether or not data isavailable from that sensor. The calibration status shows whether or notthe sensor has been calibrated. The user interface provides a method forcalibrating a sensor. In one aspect, the user interface is as describedin Ser. No. 14/742,852.

The user interface may depict an avatar image of body segments, such asthe pictures shown in FIG. 4. As a user moves a sensor, thecorresponding segment moves on the avatar tracks the sensor motion. Thisaids in identify which sensor is associated with which body segment.

The user interface may include a method for selecting a sensor based onthe sensor's associated body segment and sending a message to theselected sensor instructing it to blink its status LED. This also aidsin identifying which sensor is associated with which body segment.

The user interface may include a “Start” button which erases all RoMhistory data from the display and initiates the alignment procedure.After alignment is completed, the application begins displaying thenewly received joint rotations again.

While the exemplary user interface of FIG. 8 shows only a single jointbeing monitored, multiple joints could be monitored. The additionaljoints could be shown on additional screens, or on the same screen,although the screen would become more cluttered.

To summarize, the system described in FIG. 1 may enable a graphical userinterface (GUI) representing body joint range of motion as measuredusing IMU sensors capable of measuring their orientation relative toEarth. The conversion application 128 may supply image data to UI 132 ofthe physiologically modeled constituent joint rotation relationshipbetween the proximal body segment and distal body segment ascylindrically shaped geometric forms representing body segments. Forexample, see FIGS. 2B-2E. Instead of simplified geometric forms, thephysiologically modeled constituent joint rotation relationship betweenthe proximal body segment and distal body segment may be presented ashumanoid avatar body segments, such as shown in FIG. 7.

In another aspect, the conversion application supplies image datarepresenting the assignment of the primary IMU sensor and secondary IMUsensor to explicit body segments, as shown for example in FIG. 4.Likewise as shown in FIG. 4, the conversion application may supply imagedata representing the orientation of the primary IMU sensor andsecondary IMU sensor with respect to Earth.

As shown in FIG. 8, the conversion application may supply constituentjoint rotation data represented as a numerical value representing anextent of rotation, a constituent joint rotation relationship over aselectable period of time, or as measured with respect to a presetrotation range indicator.

FIG. 9 is a flowchart illustrating a method for measuring body jointrange of motion using IMU sensors capable of measuring theirorientations relative to Earth. Although the method is depicted as asequence of numbered steps for clarity, the numbering does notnecessarily dictate the order of the steps. It should be understood thatsome of these steps may be skipped, performed in parallel, or performedwithout the requirement of maintaining a strict order of sequence.Generally however, the method follows the numeric order of the depictedsteps. The method starts at Step 900.

Step 902 mounts a primary IMU sensor on a proximal body segment, and asecondary IMU sensor on a distal body segment connected to the proximalbody segment via a joint. Step 904 measures a sensor orientationrelationship between the primary IMU sensor and the secondary IMUsensor. In one aspect, Step 904 expresses the sensor orientationrelationship as a quaternion. Step 906 converts the sensor orientationrelationship into at least one physiologically modeled constituent jointrotation relationship between the proximal body segment and the distalbody segment. A constituent joint rotation relationship is defined in nomore than two dimensions of three-dimensional space.

In one aspect, Step 901 programmatically selects a sensor alignmentmethodology for the primary and secondary orientation sensors based uponthe body joint being measured. For example, the selection may beaccomplished using a software routine.

As noted in the explanation of FIG. 2B, Step 906 may define aconstituent axial axis rotation of the distal body segment measured withrespect to the proximal body segment, where an axial axis intersects thejoint connecting the distal body segment to the proximal body segmentand is parallel to a major axis of a smallest rectangular cuboidenclosing a body segment. A major axis is a line passing through thecenter of two opposing sides of the cuboid that are farthest apart fromeach other.

As noted in the explanation of FIG. 2C, Step 906 may define aconstituent radial axis composite rotation of the distal body segmentmeasured with respect to the proximal body segment, where the radialaxis intersects the joint connecting the distal body segment to theproximal body segment, is orthogonal to an axial axis of the distal bodysegment, and is orthogonal to an axial axis of the proximal bodysegment.

As described in the explanation of FIGS. 2D and 2E, Step 906 mayrespectively define a constituent primary radial rotation and aconstituent secondary radial rotation. The primary radial rotationrepresents a first angle measured with respect to a first axis alignedwith the radial axis of maximum possible physiologically possible radialrotation. The secondary rotation represents a second angle measured withrespect to a second axis orthogonal to the first axis and the axialaxis.

In one aspect, Step 908 displays, on a UI, the physiologically modeledconstituent joint rotation relationship between the proximal bodysegment and the distal body segment. In one variation, the constituentjoint rotation relationships are displayed as numerical valuesrepresenting an extent of rotation. In another aspect, Step 908 maps atleast one constituent rotation relationship to a physiologically definedjoint rotation, as shown for example in FIG. 6.

A system and method have been provided for measuring the range of motionof joints connecting adjacent body segments using IMU sensors. Examplesof particular algorithms and hardware units have been presented toillustrate the invention. However, the invention is not limited tomerely these examples. Other variations and embodiments of the inventionwill occur to those skilled in the art.

We claim:
 1. A method for measuring body joint range of motion usinginertial measurement unit (IMU) sensors capable of measuring theirorientations relative to Earth, the method comprising: mounting aprimary IMU sensor on a proximal body segment, and a secondary IMUsensor on a distal body segment connected to the proximal body segmentvia a joint; measuring a sensor orientation relationship between theprimary IMU sensor and the secondary IMU sensor; and, converting thesensor orientation relationship into at least one physiologicallymodeled constituent joint rotation relationship between the proximalbody segment and the distal body segment, where a constituent jointrotation relationship is defined in no more than two dimensions ofthree-dimensional space.
 2. The method of claim 1 wherein measuring thesensor orientation relationship includes expressing the sensororientation relationship as a quaternion.
 3. The method of claim 1wherein converting the sensor orientation relationship into thephysiologically modeled constituent joint rotation relationship includesdefining a constituent axial axis rotation of the distal body segmentmeasured with respect to the proximal body segment, where an axial axisintersects the joint connecting the distal body segment to the proximalbody segment and is parallel to a major axis of a smallest rectangularcuboid enclosing a body segment, and where a major axis is a linepassing through the center of two opposing sides of the cuboid that arefarthest apart from each other.
 4. The method of claim 1 whereinconverting the sensor orientation relationship into the physiologicallymodeled constituent joint rotation relationship includes defining aconstituent radial axis composite rotation of the distal body segmentmeasured with respect to the proximal body segment, where the radialaxis intersects the joint connecting the distal body segment to theproximal body segment, is orthogonal to an axial axis of the distal bodysegment, is orthogonal to an axial axis of the proximal body segment,where an axial axis intersects the joint connecting the distal bodysegment to the proximal body segment and is parallel to a major axis ofa smallest rectangular cuboid enclosing a body segment, and where amajor axis is a line passing through the center of two opposing sides ofthe cuboid that are farthest apart from each other.
 5. The method ofclaim 4 wherein defining the radial axis composite rotation comprisesdefining a constituent primary radial rotation and a constituentsecondary radial rotation, where the primary radial rotation representsa first angle measured with respect to a first axis aligned with theradial axis of maximum possible physiologically possible radialrotation, and the secondary rotation represents a second angle measuredwith respect to a second axis orthogonal to the first axis and the axialaxis.
 6. The method of claim 1 further comprising: displaying, on a userinterface (UI), the physiologically modeled constituent joint rotationrelationship between the proximal body segment and the distal bodysegment.
 7. The method of claim 6 wherein displaying the physiologicallymodeled constituent joint rotation relationship includes displaying theconstituent joint rotation relationship as a numerical valuerepresenting an extent of rotation.
 8. The method of claim 6 whereindisplaying the physiologically modeled constituent joint rotationrelationship includes mapping at least one constituent rotationrelationship to a physiologically defined joint rotation.
 9. The methodof claim 1 further comprising: programmatically selecting a sensoralignment methodology for the primary and secondary orientation sensorsbased upon the body joint being measured.
 10. A graphical user interface(GUI) representing body joint range of motion as measured using inertialmeasurement unit (IMU) sensors capable of measuring their orientationrelative to Earth, the GUI comprising: a non-transitory memory; and, aconversion application enabled as a sequence of processor executableinstructions and stored in the memory, the conversion application havingan interface to receive information from a primary IMU sensor mounted ona proximal body segment, and a secondary IMU sensor mounted on a distalbody segment connected to the proximal body segment via a joint, and aninterface to supply constituent joint rotation data of a physiologicallymodeled constituent joint rotation relationship between the proximalbody segment and the distal body segment to the display in response tomeasuring a sensor orientation relationship, where a constituent jointrotation relationship is defined in no more than two dimensions ofthree-dimensional space.
 11. The GUI of claim 10 wherein the conversionapplication supplies image data of the physiologically modeledconstituent joint rotation relationship between the proximal bodysegment and distal body segment as cylindrically shaped geometric formsrepresenting body segments.
 12. The GUI of claim 10 wherein theconversion application supplies image data of the physiologicallymodeled constituent joint rotation relationship between the proximalbody segment and distal body segment as humanoid avatar body segments.13. The GUI of claim 10 wherein the conversion application suppliesimage data representing the assignment of the primary IMU sensor andsecondary IMU sensor to explicit body segments.
 14. The GUI of claim 10wherein the conversion application supplies image data representing theorientation of the primary IMU sensor and secondary IMU sensor withrespect to Earth.
 15. The GUI of claim 10 wherein the conversionapplication supplies data representing constituent axial axis rotationof the distal body segment measured with respect to the proximal bodysegment, where an axial axis intersects the joint connecting the distalbody segment to the proximal body segment and is parallel to a majoraxis of a smallest rectangular cuboid enclosing a body segment, andwhere a major axis is a line passing through the center of two opposingsides of the cuboid that are farthest apart from each other.
 16. The GUIof claim 10 wherein the conversion application supplies datarepresenting constituent radial axis composite rotation of the distalbody segment measured with respect to the proximal body segment, wherethe radial axis intersects the joint connecting the distal body segmentto the proximal body segment, is orthogonal to an axial axis of thedistal body segment, is orthogonal to an axial axis of the proximal bodysegment, where an axial axis intersects the joint connecting the distalbody segment to the proximal body segment and is parallel to a majoraxis of a smallest rectangular cuboid enclosing a body segment, andwhere a major axis is a line passing through the center of two opposingsides of the cuboid that are farthest apart from each other.
 17. The GUIof claim 16 wherein the conversion application supplies datarepresenting a constituent primary radial rotation and a constituentsecondary radial rotation, where the primary radial rotation representsa first angle measured with respect to a first axis aligned with theradial axis of maximum possible physiologically possible radialrotation, and the secondary rotation represents a second angle measuredwith respect to a second axis orthogonal to the first axis and the axialaxis.
 18. The GUI of claim 10 wherein the conversion applicationsupplies constituent joint rotation data represented as a numericalvalue representing an extent of rotation.
 19. The GUI of claim 10wherein the conversion application supplies constituent joint rotationdata representing a constituent joint rotation relationship over aselectable period of time.
 20. The GUI of claim 10 wherein theconversion application supplies constituent joint rotation datarepresenting a constituent joint rotation relationship as measured withrespect to a preset rotation range indicator.
 21. A system for measuringbody joint range of motion using inertial measurement unit (IMU) sensorscapable of measuring their orientation relative to Earth, the systemcomprising: a primary IMU sensor having a wireless interface, formounting on a proximal body segment; a secondary IMU sensor having awireless interface, for mounting on a distal body segment connected tothe proximal body segment via a joint; a joint rotation measurementdevice comprising: a processor; a non-transitory memory; a communicationmodule having a wireless interface to communicate with the primary IMUsensor and secondary IMU sensor, and an internal interface; and, aconversion application connected to the internal interface, enabled as asequence of processor executable instructions, and stored in the memory,the conversion application measuring a sensor orientation relationshipbetween the primary IMU sensor and the secondary IMU sensor, andconverting the sensor orientation relationship to supply physiologicallymodeled constituent joint rotation relationship data describing therelationship between the proximal body segment and the distal bodysegment, where a constituent joint rotation relationship is defined inno more than two dimensions of three-dimensional space.
 22. The systemof claim 21 wherein the conversion application measures the sensororientation relationship by expressing the sensor orientationrelationship as a quaternion.
 23. The system of claim 21 wherein theconversion application supplies data defining a constituent axial axisrotation of the distal body segment measured with respect to theproximal body segment, where an axial axis intersects the jointconnecting the distal body segment to the proximal body segment and isparallel to a major axis of a smallest rectangular cuboid enclosing abody segment, and where a major axis is a line passing through thecenter of two opposing sides of the cuboid that are farthest apart fromeach other.
 24. The system of claim 21 wherein the conversionapplication supplies data defining a constituent radial axis compositerotation of the distal body segment measured with respect to theproximal body segment, where the radial axis intersects the jointconnecting the distal body segment to the proximal body segment, isorthogonal to an axial axis of the proximal body segment, is orthogonalto an axial axis of the distal body segment, where an axial axisintersects the joint connecting the distal body segment to the proximalbody segment and is parallel to a major axis of a smallest rectangularcuboid enclosing a body segment, and where a major axis is a linepassing through the center of two opposing sides of the cuboid that arefarthest apart from each other.
 25. The system of claim 24 wherein theconversion application supplies data defining a constituent primaryradial rotation and a constituent secondary radial rotation, where theprimary radial rotation represents a first angle measured with respectto a first axis aligned with the radial axis of maximum possiblephysiologically possible radial rotation, and the secondary rotationrepresents a second angle measured with respect to a second axisorthogonal to the first axis and the axial axis.
 26. The system of claim21 further comprising: a user interface (UI) connected to the conversionapplication to display information representing the physiologicallymodeled constituent joint rotation relationship between the proximalbody segment and the distal body segment.
 27. The system of claim 26wherein the UI displays the constituent joint rotation relationship as anumerical value representing an extent of rotation.